The present invention relates generally to optical devices and, more particularly, to optical communication devices including a superconductive assembly in a data transmission scheme with low extinction ratio.
Developments in optical communication devices are driving the speed of optical communication systems ever faster. For example, high speed light modulators and data links based on superconductive materials have been demonstrated by Puzey in U.S. Pat. No. 5,768,002, U.S. Pat. No. 5,886,809, U.S. Pat. No. 6,115,170, copending U.S. patent application Ser. No. 09/637,098, now U.S. Pat. No. 6,285,487, and copending U.S. Patent Application Ser. No. 09/815,972 Attorney Docket Number PUZ-P001CIP. All of the aforementioned patents and patent applications of Puzey are co-assigned assigned with the present application and are incorporated herein by reference. Optical devices based on superconductive materials are capable of operating at high data rates such as, for example, transmitting optical data signals at data rates of terabits per second (Tbit/s) at a given wavelength over a single optical fiber.
As described in the aforementioned Puzey patents and patent applications, a superconductive material is in a superconducting state when the current density in the material, magnetic field around the material and temperature of the material are below certain critical values. In the superconducting state, the superconductive material exhibits properties of an essentially perfect conductor. The electrical resistance of the material is substantially zero in the superconducting state, and the material reflects electromagnetic radiation with high reflectivity. When any of the above conditions is not met, such that the current density, magnetic field or temperature rises above its respective critical value, the material no longer behaves as a superconductor and is in a normal (non-superconducting) state. In the normal state, the material behaves in a manner similar to that of semiconductor materials in that some portion of an incident electromagnetic radiation is transmitted therethrough. The change in the electromagnetic properties of the superconductive material is used in optical devices to produce, for example, the high speed optical modulators of Puzey.
One of the factors limiting the use of superconductors in device applications resides in the difficulty in achieving a low extinction ratio in the modulated optical signal. Extinction ratio of an optical device is defined as a ratio of a first optical signal intensity produced by the device, when the device is nominally producing a null signal, to a second optical signal intensity produced by the device, when the device is intended to be producing a non-zero optical signal. For example, the extinction ratio of an optical device based on a superconductive material can be defined as the ratio of a first signal intensity of the device, when the superconductive material is in the superconducting state, to a second signal intensity, when the superconductive material is in the normal state. Using prior art optical modulators using a single superconducting film, such as those described in the aforementioned Puzey patents, the Applicants have achieved extinction ratios of at most 1:4. Current industry standards require an extinction ratio of at least 1:20 for a optical device that produces a digital, optical signal.
The present invention provides optical communication devices which serve to resolve the problems described above with regard to prior art optical communication devices based on superconductive materials in a heretofore unseen and highly advantageous way and which provides still further advantages.
As will be described in more detail hereinafter, there is disclosed herein an optical assembly for modulating input light and providing modulated light at an output thereof is disclosed. The optical assembly includes a first arrangement, which in turn includes a layer of superconductive material having at least a part of the input light incident thereon as incident light. The superconductive material is switchable between a first state, in which the superconductive material exhibits a first refractive index, and a second state, in which the superconductive material exhibits a second refractive index. The first arrangement is configured to direct to the output as the modulated light a first fraction of the incident light, when the superconductive material is in the first state, and a second fraction of the incident light, when the superconductive material is in the second state, such that the modulated light exhibits a given value of extinction ratio. The extinction ratio is defined as a ratio of the first fraction of the incident light to the second fraction of the incident light at the output. The optical assembly further includes a second arrangement cooperating with the first arrangement to provide at the output of the optical assembly another value of the extinction ratio that is smaller than the given value.
In another aspect of the invention, the superconductive material of the first aspect of the invention is additionally configured to selectively modulate the incident light such that the superconductive material, when in the first state, sets the modulated light to a first polarization state, and, when in the second state, sets the modulated light to a second polarization state. The second arrangement includes a polarizing arrangement for setting the input light to a known, initial polarization state, and an analyzer configured to receive the modulated light and to selectively absorb or reflect substantially all of the modulated light of the first polarization state while transmitting at least a portion of the modulated light of the second polarization state therethrough.
In still another aspect of the invention, the incident light of the first aspect of the invention has a particular wavelength and the superconductive material exhibits a first reflectance in the first state and a second reflectance in the second state. The second arrangement in this aspect includes a reflective surface positioned substantially parallel to and spaced apart from the layer of superconductive material such that the reflective surface cooperates with the layer of superconductive material to form an etalon. The etalon is configured to block or reflect substantially all of the incident light of the particular wavelength, when the superconductive material is in the first state, and to direct the incident light of the particular wavelength to the output, when the superconductive material is in the second state.
In yet another aspect of the invention, the superconductive material of the first aspect is further configured such that the incident light, which is incident on a surface of the layer of superconductive material, emerges as the modulated light at a first spot on an opposite surface of the layer of superconductive material in the first state, and at a distinct, second spot on the opposite surface of the layer of superconductive material in the second state. The second arrangement in this aspect includes a light directing arrangement for directing the modulated light to the output.
In a further aspect of the invention, the superconductive material of the first aspect is additionally configured to reflect the incident light such that the superconductive material, when in the first state, produces the modulated light with a first phase lag and, when in the second state, produces the modulated light with a second phase lag. The second arrangement of this aspect includes a polarizing arrangement for setting the input light in a known, initial polarization state, and a compensator configured to compensate for the first phase lag such that, when the superconductive material is in the first state, the modulated light is linearly polarized while, when the superconductive material is in the second state, the modulated light is elliptically polarized after being acted upon by the compensator. The second arrangement further includes a directing configuration, which in turn includes a directing arrangement and a dielectric plate. The directing arrangement is configured to direct the modulated light to the dielectric plate. The dielectric plate is oriented such that, when the superconductive material is in the first state, the modulated light is incident on the dielectric plate at Brewster""s angle and the modulated light is transmitted substantially unaltered through the dielectric plate while, when the superconductive material is in the second state, at least a portion of the modulated light is reflected by the dielectric plate to the output.
In a still further aspect of the invention, a method for modulating input light and providing modulated light at an output is disclosed. The method includes the step of directing at least a part of the input light as incident light at a layer of superconductive material. The superconductive material is switchable between a first state, in which the superconductive material exhibits a first refractive index, and a second state, in which the superconductive material exhibits a second refractive index. The method further includes the step of arranging the superconductive material such that, when the superconductive material is in the first state, a first fraction of the incident light is directed to the output as modulated light and, when the superconductive material is in the second state, a second fraction of the incident light is directed to the output as modulated light. The modulated light has a given value of extinction ratio, which is defined as a ratio of the first fraction of the incident light to the second fraction of the incident light at the output. The method also includes the step of acting on at least one of the input light and the first and second fractions of incident light so as to provide at the output another value of the extinction ratio that is smaller than the given value.